Juvenile Paget's disease (JPD), an autosomal recessive osteopathy, features rapidly remodeling woven bone, osteopenia, fractures, and progressive skeletal deformity that can be fatal by young adult life. Homozygous deletion of the TNFRSF11B gene, encoding osteoprotegerin (OPG), was identified as the cause of JPD in two Navajo families. Previously, a defect in the RANK (receptor activator of nuclear factor ?-B) gene was determined to be the cause of a related disorder, familial expansile osteolysis (FEO). Hence, mutations in genes involved in the OPG/RANKL/RANK/NF-?B signaling pathway are responsible for JPD and related disorders of bone turnover. The long-range goal of this study is to identify other mutations in members of the OPG/RANKL/RANK/NF-?B pathway that cause JPD and related diseases in newly-ascertained patients, and to use in vitro cell modeling systems to assess the downstream effects of these mutations, to elucidate the pathobiology of these disorders. The Specific Aims of this project are: Specific Aim 1: Ascertain and evaluate additional patients worldwide with juvenile Paget's disease (JPD), familial expansile osteolysis (FEO), and related disorders. Specific Aim 2: In new patients with JPD, FEO, and related disorders, identify the genetic defects that cause these skeletal diseases. Specific Aim 3: Characterize the downstream effects of mutant OPG, RANK, and other members of the OPG/RANKL/RANK/NF-?B signaling pathway on osteoclast formation and action. Additional patients will be ascertained, and evaluated radiographically, biochemically, and molecularly. DNA and RNA samples will be screened for mutations in TNFRSF11B, encoding OPG, TNFRSF11A, encoding RANK, and TNFSF11, encoding RANKL, using PCR and capillary DNA sequencing, and RT-PCR. We will also use a microarray-based copy number analysis to identify potential genomic defects in these patients, and identify new candidate genes. For patients without mutations in the OPG, RANK, or RANKL genes, large-scale Solexa sequencing will be used to search other candidate genes in the OPG/RANKL/RANK/NF-?B signaling pathway for disease-causing mutations, including SQSTM1, VCP, TRAF6, aPKC, NIK, and others. Modulators of RANK signaling will also be examined, including TRAIL, IL-1, MCSF, c-FMS and additional cytokines and their cognate receptors. In vitro osteoclast culture assays will be used to determine downstream effects of mutations on osteoclast formation and function. These studies will further elucidate the genetic bases for JPD, FEO, and related disorders of bone turnover, caused by defects in the OPG/RANKL/RANK/NF-?B signaling pathway, which is critical for osteoclast formation and action.